The continuing evolution of microprocessor control has afforded increasingly sophisticated vehicular control systems. improvements in hardware, including greater memory capacity and faster microprocessors, have facilitated implementation of complex control strategies. In particular, engine control strategies have become more sophisticated to accommodate the various conditions encountered during normal engine operation.
One particularly challenging control function is that of idle speed control (ISC). A number of constraints are placed on the control of engine idle speed, including maintaining satisfactory fuel economy, meeting emissions requirements, and maintaining acceptable driveability. Variations in idle speed are particularly noticeable to vehicle occupants since the engine is operating at a relatively low speed and external distractions, such as road noise or wind noise, are typically negligible or minimal. Furthermore, the low operating speed of the engine produces a relatively low amount of available power at a time when accessory load may be at its highest level. For example, power steering demand is greater while the vehicle is stationary or is slowly moving than when traveling at highway speeds. Similarly, many accessories may be operated shortly after starting a vehicle as the driver adjusts the vehicle environment for his or her preferences. These accessories may include the headlamps, air conditioning or defrost, power windows, power seats, and lights. Shifting an automatic transmission from park to reverse or drive also imposes a load on the engine.
Preferably, the engine control system will maintain a substantially constant idle speed while being subjected to various disturbances associated with operation of numerous engine accessories. In addition, it is important to avoid engine stall as a result of an unexpected load on the engine.
Prior art ISC strategies react to load torques only after the occurrence of the disturbance. Some systems attempt to anticipate the occurrence of a load disturbance without accounting for the engine system dynamics, which may result in undesirable idle speed variations. Other prior art systems utilize feed-forward control strategies so that the control actions start concurrently with (but not before) the start of the torque disturbance. Although this strategy improves the response time of the control system, it is still susceptible to noticeable variations of engine idle speed.